228 GEOLOGICAL UKSTOUY OF LAKE LAHONTAN. 



has been made without consider! uy the amoiiiit of saline matter brought 

 into the hikes by springs ; and assuuies that no salts remained in the basin 

 when the process began. We know, however, that very large quantities of 

 various salts are contributed to the lakes of the Lahontan basin from sub- 

 terranean sources. Any conclusion derived from the above considerations 

 must be weighted by the fact that the analysis of the rivers were in all 

 cases of samples collected outside the old lake basin, and therefore iu;)t 

 affected by the substances derived from the richly saline clays and marls of 

 Lahontan date, through which they carved channels sometimes a hundred 

 miles in length before reaching the lakes into which they empty. The 

 present lakes also derive large quantities of foreign matter from the tem- 

 porary rills that are formed after the infrequent storms and are charged with 

 the salts derived from the efflorescences formed on the surrounding desert 

 surfaces during the arid season. It is true that the data we have used for 

 computing the flow of the rivers as well as for obtaining the average annual 

 evaporation are based upon very incomplete observations, but we feel con- 

 fident that future study will show these estimates to be below rather than 

 above the reality. With all these considerations in view, it seems evident 

 from the calculations we have made, that the lakes of the Lahontan basin 

 could not have existed under the jjresent conditions for more than a few 

 centuries at the most without being far more saline and alkaline than we 

 now find them. From the hypothesis of the freshening of lakes by desic- 

 cation we conclude that the basin of Lake Lahontan was completely desic- 

 cated for n, ])eriod, ending, we will say, about three hundred years ago, which 

 was sufficiently long to allow of the burial of any saline deposits that may 

 have been left from the evaporation of previous water-bodies in the same 

 basin. By complete desiccation we mean that the various secondary basins 

 fornierlv flooded by Lake Lahontan became sufficiently dry during success- 

 ive years, or at intervals of a number of years, to admit of the formation 

 of playa-lakes and playas, and the bui-ial and absori)tion of saline mattei- 

 beneath playa deposits. That this was the actual condition of the various 

 valleys composing the Lahontan basin at no distant date is rendered still 

 more probable by the fact that the bottoms of all the lakes of th_e region 



